Photocatalysts such as titanium oxide have been recently utilized in various applications such as exterior materials for buildings. Employment of the photocatalyst makes it possible to harness light energy to decompose various types of noxious substances and to hydrophilize the surface of a substrate coated with the photocatalyst to allow a stain deposited on the surface to be easily washed away with water. The following techniques have been known for producing photocatalyst-coated bodies coated with such a photocatalyst.
It is known to use an aqueous dispersion comprising photocatalytic metallic oxide particles, a colloidal silica, and a surfactant to impart hydrophilic properties to the surface of a synthetic resin or the like (see, for example, Japanese Patent Laid-Open Publication No. 1999-140432). In this technique, the hydrophilic properties are intensified by adding a large amount of a surfactant ranging from 10 wt % to 25 wt %. Also, the film thickness is set at 0.4 μm or less in order to prevent white turbidity from being caused by diffuse reflection of light.
It is also known to form on the substrate a coating film comprising a photocatalytic titanium dioxide and a binder silica sol to obtain a photocatalyst body (see, for example, Japanese Patent Laid-Open No. 1999-169727). In this technique, the additive amount of the silica sol in view of SiO2 is 20 parts to 200 parts by weight of the titanium dioxide, and the TiO2 content ratio is high. The particle diameter of the silica sol is as small as 0.1 nm to 10 nm.
It is also known that a photocatalyst coating material is used to form a photocatalyst coating film that transmits 50% or more of light having a wavelength of 500 nm and blocks 80% or more of light having a wavelength of 320 nm (see, for example, in Japanese Patent Laid-Open No. 2004-359902). In this technique, an organosiloxane partial hydrolysate is used as a binder of the photocatalyst coating material, in which the organosiloxane partial hydrolysate is contained preferably in an amount of 5 mass % to 40 mass % of the entire coating composition.
In the meantime, a problem has been conventionally known that, when a substrate for a photocatalyst layer is composed of an organic material, the organic material is decomposed or deteriorated due to photocatalytic activity of the photocatalyst. In order to address this problem, it is known that an adhesive layer made of a silicone-modified resin or the like is provided between a photocatalyst layer and a substrate to protect the substrate from being deteriorated by the photocatalysis (see, for example, WO97/00134).